nixos/wrappers: enforce target executable is ELF

This *will* break some things, such as `nixosTests.espanso`.
This is intended: Loading bash, python or perl scripts via SUID or extensive capabilities is dangerous.
PERL5LIB, PYTHONPATH or BASH_ENV make these things trivial LPE primitives.

The mime types being matched include both static, non-static and pie ELFs.
This should be fine until we somehow get yet another type of magic and associated mime type.

nixos/modules/security/wrappers/default.nix

@@ -361,16 +361,19 @@ in
 
     ###### wrappers consistency checks
     system.checks = lib.singleton (
-      pkgs.runCommandLocal "ensure-all-wrappers-paths-exist"
+      pkgs.runCommandLocal "ensure-wrapper-integrity"
         {
-          nativeBuildInputs = [ pkgs.libcap-text-verifier ];
+          nativeBuildInputs = [
+            pkgs.libcap-text-verifier
+            pkgs.file
+          ];
           preferLocalBuild = true;
         }
         ''
           # make sure we produce output
           mkdir -p $out
 
-          echo -n "Checking that Nix store paths of all wrapped programs exist... "
+          echo -n "Checking that Nix store paths of all wrapped programs exist and are ELF executables... "
           ${lib.toShellVar "wrappers" (lib.mapAttrs (n: v: v.source) wrappers)}
           for name in "''${!wrappers[@]}"; do
             path="''${wrappers[$name]}"
@@ -382,6 +385,15 @@ in
               test -t 1 && echo -ne '\033[0m'
               exit 1
             fi
+            magic=$(file --mime --brief --dereference "$path")
+            if ! [[ "$magic" =~ application/x-executable || "$magic" =~ application/x-pie-executable ]]; then
+              test -t 1 && echo -ne '\033[1;31m'
+              echo "FAIL"
+              echo "The target executable $path is not an ELF! This is a security risk."
+              echo "Script wrappers (e.g. bash or python) are commonly susceptible to dangerous env var injections."
+              test -t 1 && echo -ne '\033[0m'
+              exit 1
+            fi
           done
           echo "OK"